In 1919, a British Royal Navy ship, the H.M.S. Polar Queen, delivers a small expedition to the coast of an “ice continent” near Antarctica called Caprona. The party, led by American Major Ben McBride, is searching for his boyhood friend, Bowen Tyler, who was marooned on the island in 1916 and tossed a canister containing a manuscript into the ocean, hoping he would be rescued. Other members of the expedition include airplane mechanic Hogan, who served under McBride in World War I, Dr. Norfolk, a British biologist, and Lady “Charly” Charlotte, a photojournalist whose London, England, newspaper financed the adventure. Caprona, populated by cavemen and dinosaurs, is surrounded by an enormous “ice wall” and McBride flies the party over it to the tropical interior in the Amphibian, a seaplane. A Pterodactyl attacks the Amphibian and McBride is forced to crash land. Meanwhile, the Polar Queen loses radio contact with the expedition, but Captain Lawton vows to wait three to four weeks for their return. Meanwhile, Hogan assesses the damage to the Amphibian as the others encounter a Stegosaurus, which they use to winch the plane to level ground. Hogan estimates it will take several days to repair, but McBride feels they cannot spare the time. McBride, Norfolk and Charly proceed on foot and find a young primitive woman, Ajor, who carries a Bowie knife, speaks English, and knows Tyler. She tells Charly that she is a member of the Galu tribe. Several years earlier, Tyler and a woman named Lisa, the lone survivors of the previous expedition, arrived and befriended them. All was peaceful until four months ago, when the Na-ga, a more advanced tribe, attacked and slaughtered most of the Galu. Ajor escaped and has been on the run ever since. Tyler and Lisa may have been captured, but the Na-ga has a practice of sacrificing hostages to the volcano god, Nagramata. As Ajor leads the others to the Na-ga, they witness another tribe, the Band-lu, hunting two Galu men. The Galu are felled by arrows, but not before one tells Ajor that Tyler was taken with some others to the Mountain of the Skulls, where the Na-ga live. As Ajor goes ahead to look for a shortcut through the mountain, McBride, Norfolk, and Charly are attacked and captured by the Band-lu, who stake them to the ground. Ajor rescues them and leads them through the cavern to the other side of the mountain, where horseback-riding Na-ga warriors meet them. Their leader speaks English and explains that Tyler taught it to him, and offers to take McBride and company to Tyler. They reach the Mountain of Skulls and are introduced to the Na-ga leader, Sabbala. The men are separated from the women, and Ajor and Charly are told that they are to be brides of Nagramata and sacrificed at sunrise. McBride and Norfolk are led away to a cell, where they find Tyler imprisoned. He tells them that Sabbala killed Lisa. Ambushing the Na-gas who bring them food, the three men fight their way into Sabbala’s chamber to rescue Ajor and Charly. Sabbala falls into the adjoining volcano, triggering an eruption, but McBride, Tyler, Norfolk, Ajor, and Charly are forced to escape through the volcano. They reach the surface only to be pursued by more Na-ga. Tyler warns McBride that even if they escape, they will never get off the island as it is alive and the volcano is its heart. McBride and Tyler hold off the Na-ga, while Norfolk, Ajor, and Charly enter another cave. A Na-ga kills Tyler with an arrow as the volcano continues to erupt. McBride rejoins the others and they narrowly escape a dinosaur. Back at the plane, Hogan attempts to finish repairs as volcanic explosions buffet the aircraft. On the coast, the Polar Queen waits, but Lt. Whitby informs Captain Lawton that they must pull out immediately or be crushed by an iceberg. Meanwhile, on the island, McBride and his party flee through Band-lu territory, avoiding the ground explosions caused by the volcano. They reach Hogan and the plane takes off. As they reach the ice wall, they are forced to lessen the weight of the plane by throwing anything extraneous overboard, including Charly’s camera equipment. As the Amphibian clears the highest peak it loses power and McBride glides the plane to a rough landing near the Polar Queen. McBride, Charly, Hogan, Ajor, and Norfolk, are rescued and toasted by Lawton and his crew as the ship sails to safety.